This invention generally relates to tooling for removing and installing a control rod drive from the drive housing mounted in the vessel of a boiling water reactor, and is specifically concerned with a compact and lightweight installation and removal assembly capable of expeditiously and remotely performing a control rod drive installation and removal operation without the necessity of providing special support structures in the undervessel cavity.
Tooling systems and methods for removing and installing the control rod drives from the drive housings of boiling water reactors are known in the prior art. Some of these tooling systems include a truck or carriage to which a beam is pivotally mounted. When the beam is swung into a horizontal position, the combination of the carriage and beam can be rolled along the service rails normally present in the undervessel cavity located beneath the reactor vessel. The beam is provided with a bucket for capturing an end of a control rod drive assembly, as well as a lifting and lowering mechanism for moving this bucket up or down when the beam is in a vertical position beneath a control drive housing. Examples of such tooling systems are disclosed in U.S. Pat. Nos. 4,288,290, 4,292,133, and Japanese patent 29,596.
While pivoting-beam type tooling systems have met with some success in installing and removing the control drive rods of boiling water reactors, the applicants have observed that each of these prior art systems has a number of operational shortcomings. However, before these shortcomings can be appreciated, some background as to the environment where these tooling systems are used is necessary.
Boiling water reactors generally include a cylindrically shaped reactor vessel which is supported over a cylindrically-shaped concrete room called the undervessel cavity in the art. Extending down from the bottom of the reactor vessel is an array of tube-like housings for housing the control rod drives that slide control rods up and down within the fuel assemblies disposed within the reactor vessel in order to control the fission reaction which occurs therein. Over a period of time (which is typically approximately four years) the bushings and seals of the control rod drives begin to wear out, thereby necessitating their replacement. The principle purpose of the undervessel cavity disposed beneath the reactor vessel is to provide access to the control rod drives and other reactor components extending downwardly from the bottom of the reactor vessel so that they may be serviced. Such undervessel cavities are typically provided with a pair of service rails which allow maintenance equipment to be easily shuttled across the diameter of the cylindrically-shaped undervessel cavity. To allow such maintenance equipment to be positioned at any given point under the reactor vessel, the ends of these service rails include wheels which engage a circular track that circumscribes the inner wall of the undervessel cavity. Hence a maintenance device may be moved in a polar-coordinate fashion under the reactor vessel by traversing the device to a selected point along the service rails and by rotating these service rails from zero to 360 degrees until the device is disposed under the housing of a selected control rod drive or other component.
Unfortunately, the undervessel cavity provides very little clearance for the entrance and operation of control rod drive installation and removal systems. While the bottom ends of the housings for the control rod drives are almost seven feet from the top of the service rails, the actual usable clearance is often only about four feet above the service rails due to the large number of delicate instrument tubes which extend from the bottom of the reactor vessel, and further due to the "forest" of electrical cables used to power the control monitors and control rod drives which drape down from the bottom of the vessel.
The applicants have noted that the tooling systems developed thus far for the removal and installation of such control rod drives suffer from a number of deficiencies which could bear improvement. These systems must be manually wheeled out onto the service rails, thus exposing workers to the "shine" of radiation emitted by the reactor vessel. Some of these systems use chain and sprocket drive mechanisms for elevating the control rod drives into position which can damage or completely cut through any of the maze of instrumentation tubes and electrical cables which hang down from the bottom of the reactor vessel. The operation of such tooling systems must be very carefully monitored by maintenance personnel standing in the immediate proximity to insure that none of the moving chains and sprockets damages any of the reactor components. The long chains such systems are further prone to stretching, which makes the automatic operation of these machines difficult as the number of sprocket turns necessary to elevate a particular control rod drive can vary. Others of these tooling systems are multi-component systems which include separate control rod elevating mechanisms or bolt removal assemblies that necessitate the installation of special tracks within the undervessel cavity. Some of these systems are considerably heavier than the existing service rails can carry, thereby necessitating replacing these rails. The installation of additional tracks and the replacement of the existing service rails again adds substantiallY to the time required to remove and replace worn control drive housings. Further, the pivotal stroke of the beams of these systems is very often larger than the clearance afforded within the undervessel cavity at a given drive housing, which necessitates manually moving the carriage of the device as the carrying beam is pivoted to avoid mechanical interference between the ends of the pivoting beam and one or more of the instrument tubes, electrical cables and other reactor components. Such manual positioning and repositioning of the carriage on the service rails greatly protracts the operational time required to either remove or install a control rod drive, which has the effect of requiring the maintenance personnel operating the system to spend substantial amounts of time in the radioactive undervessel cavity. Finally, the control rod drive lifting mechanisms associated with the pivoting beams provide no means for facilitating a rapid alignment between a control rod drive and a particular housing, and additionally are not completely reliable in operation. All of these are significant drawbacks that necessitate a great deal of manual labor in a highly radioactive environment.
Clearly, there is a need for a control rod drive installation and removal system that is sufficiently lightweight and compact in structure so that is may be used solely in conjunction with the service rails already provided in the undervessel cavity, and whose operational movements are short and directed either within or under the carriage of the system so as to avoid mechanical interference with the reactor components. Moreover, the system should be automatically and remotely operable, and the pivoting stroke of the beam of the tool should be short enough to eliminate or at least minimize the necessity for multiple movements of the carriage along the service rails whenever the support beam is pivoted. Ideally. such a system should further provide a self-contained lifting and lowering mechanism which is capable of moving a control rod drive from a position at the bottom end of the carriage to a position completely installed within a drive housing without the necessity of adding additional elevating mechanisms to the cradle. Finally, such a system should have a means for facilitating the rapid alignment of the end of a control rod drive with the open end of a drive housing so as to expedite the operation of the system and to minimize the exposure of the system operators to potentially harmful radiation.